...
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The URI used as a system identifier with the public identifier allows
the user agent to download the DTD and entity sets as needed.
The FPI for the Transitional HTML 4.01 DTD is:
"-//W3C//DTD HTML 4.01 Transitional//EN"
This version of the transitional DTD is:
http://www.w3.org/TR/1999/REC-html401-19991224/loose.dtd
If you are writing a document that includes frames, use
the following FPI:
"-//W3C//DTD HTML 4.01 Frameset//EN"
This version of the frameset DTD is:
http://www.w3.org/TR/1999/REC-html401-19991224/frameset.dtd
Use the following (relative) URIs to refer to
the DTDs and entity definitions of this specification:
"strict.dtd"
"loose.dtd"
"frameset.dtd"
"HTMLlat1.ent"
"HTMLsymbol.ent"
"HTMLspecial.ent"
-->
%HTMLlat1;
%HTMLsymbol; -->
%HTMLspecial; -->
]]>
]]>